The invention relates to a camera tube for thermal imaging and having an entrance window consisting of an electrically conductive material, a target for converting thermal images, and an electron beam system for scanning the target. A mechanically moved thermal radiation modulator is arranged in front of the entrance window.
In a camera tube for thermal imaging with an insulated target, as is the case, for example in a pyroelectric camera tube, the incident heat radiation is periodically chopped by means of a mechanical modulator. This is required for good image resolution and for producing stationary images. Normally, the modulator consists of a periodically rotating shutter with a light period and a dark period. It is situated right in front of the entrance window of the camera tube for thermal imaging.
Such an arrangement is known and described, for example, in U.S. Pat. No. 3,774,043, incorporated herein by reference. The entrance window consists of an insulating material and exhibits an electrically conductive layer on the inside which is electrically connected to a signal electrode layer on the pyroelectric target.
It is likewise known, for example, from the periodical "IEEE Transactions on Electron Devices", Vol. ED-21, No. 8, Aug. 1974, Pages 462-469, incorporated herein by reference, to make the entrance window of germanium for thermal rays of from 8 to 14 .mu.m wavelength. The target is mounted behind and on the entrance window in an electrically conductive connection. The germanium is itself likewise electrically conductive. Silicon, which is likewise electrically conductive, is employed for smaller wavelengths in the so-called second atmospheric window.
In operation, capacitive image disruptions are produced on the signal electrode via these conductive entrance windows from the modulator positioned right in front of the window. Arrangements can be obtained in which openmeshed, monofil or single filament grids for electrical screening or shielding are provided between the entrance window and the modulator in order to prevent such pick-ups. The scatter and absorption at such grids, however, diminish good image quality.